Bugfixes from audacity

Audacity has a copy of portmidi sources, I think this patch would be interesting to use here:

https://github.com/audacity/audacity/commit/6fa857c3ae6643706015a143f1325e05f53ecdde

(More specifically, the modifications of `lib-src/portmidi/pm_mac/pmmacosxcm.c`)